A construction of infrared intrusion detectors, i.e., infrared radiation detectors intended to produce an alarm upon the detection of an intruding human, poses both optical and detector fabrication problems. It is frequently desired to cover a 90.degree. field of view, for example when the detector is placed in the corner of a room. Merely placing an infrared radiation detector in the room is infeasible. There must in addition be a focusing mechanism to gather infrared radiation from the intruder, wherever he or she might be, and focus this radiation on the infrared sensing element. If the spherical mirror is used to cover a 90.degree. field of view, the detector must be large enough to cover this 90.degree. field of view. For a low energy system, such as an intruding person entering the field of view of the detector, the energy received by the infrared radiation detecting element must be chopped and amplified many times in order to provide a sufficiently low detection limit that a human intruder can be detected. This approach would require a large and complex system. The required detector element size would then have to be larger than practical. This is because the image plane of the spherical mirror is itself a spherical surface with the same radius of curvature as the focal length of the mirror (i.e., one-half the radius of curvature), with the same center as the mirror.
Copending, commonly assigned, U.S. patent application Ser. No. 426,144, filed Dec. 19, 1973, by William R. Harding, now U.S. Pat. No. 3,923,382 provides a partial solution to this problem, by providing a multifaceted mirror for gathering infrared radiation from a plurality of discrete, spaced apart fields of view, and focusing this radiation on a single sensing element. The multifaceted mirror of application Ser. No. 426,144 has a concave surface, this concave surface comprising a plurality of radiation gathering surface portions. Each of the radiation gathering surface portions of the mirror of application Ser. No. 426,144 is itself concave, has a focal length equal to that of the focal length of the other concave radiation gathering surface portions, and is placed in such a configuration that all of the concave surface portions have a common focal point. The shapes of the concave radiation gathering surface portions of the mirror of application Ser. No. 426,144 are preferably spherical polygons (such as hexagons), contiguously arranged so that their focal points are all at the sensing element of the radiation detector. This arrangement is known as "on axis" optics, since the point to which radiation is directed by the individual radiation gathering surface portions of the multifaceted mirror is located at the focal point of each radiation gathering surface portion, i.e., on the axis of each individual facet. The radiation detecting element thus casts a shadow in the center of the field of view emanating from each individual radiation gathering surface portion of the multifaceted mirror.
While the configuration contemplated by application Ser. No. 426,144 is advantageous and in many cases represents a highly desirable mirror facet configuration for the intended use of the radiation detector, it has been found that by making certain modifications in the mirror facet configuration, an improved multifaceted mirror structure for some applications can be produced.
It is, therefore, an object of the present invention to provide an improvement upon the preferably polyhexagonal multifaceted mirror structure of copending application Ser. No. 426,144, to obtain a more efficient use of the available area of the multifaceted mirror.